1. Field of the Invention
The present invention relates to an electric power steering device, in particular, to an electric power steering device for exerting power of an electric motor on a vehicle steering system to reduce a steering force.
2. Description of the Related Art
As described in, for example, Japanese Patent No. 2506269, a conventional electric power steering device includes failure detection means for monitoring a PWM signal of electric-motor driving means and determining the occurrence of an abnormality in the electric power steering device based on the PWM signal to generate an abnormality signal. When the abnormality occurs, a relay circuit is interrupted based on the abnormality signal so as to stop power supply to the electric-motor driving means.
As described above, the electric power steering device described in Japanese Patent No. 2506269 includes the above-mentioned failure detection means which interrupts the relay circuit based on the abnormality signal so as to stop the power supply to the electric-motor driving means. Therefore, the electric power steering device can be protected from every abnormality which affects the PWM signal.
In Japanese Patent No. 2506269, the following fail-safe power relay is proposed. The fail-safe power relay is provided between a power source for a vehicle and the electric-motor driving means to stop the power supply to the electric-motor driving means in case of occurrence of an abnormality.
In recent years, noise reduction of on-vehicle electronic devices has been demanded. However, Japanese Patent No. 2506269 has a problem in that no measure for noise-reduction has been taken.
In the automobile industry of recent years, a method of gradually performing a mechanical relay-OFF operation as the measures for noise-reduction has been proposed. According to the method, however, the mechanical relay is gradually interrupted in a state in which the mechanical relay is energized even when an abnormality occurs in the electric power steering device. Therefore, an arc discharge occurs between contacts of the mechanical relay to disadvantageously lower contact welding resistance. Therefore, the above-mentioned method has much restriction on an effective interrupting-current capacity. Thus, the application of the above-mentioned method to high-output products is limited. Therefore, the above-mentioned method has a problem of lack in practicality. Moreover, when the contact welding occurs, there is a fear in that the stop of power supply to the electric-motor driving means in case of occurrence of the abnormality, which is expected as a function of the mechanical relay of the electric power steering device, cannot be performed.